The present disclosure relates to photoreceptors, and methods for making and using such photoreceptors, which photoreceptors are positively chargeable and provide a long useful life. More particularly, the disclosure relates to photoreceptors having, in order, at least a substrate layer, a charge transport layer, a charge generating layer, and a charge transporting or photoconductive overcoat layer.
In electrophotography, also known as Xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and other materials. In addition, the imaging member may be layered in which each layer making up the member performs a certain function. Current layered organic imaging members generally have at least a substrate layer and two electro or photo active layers. These active layers generally include (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing charge transport molecules or materials. These layers can be in a variety of orders to make up a functional device, and sometimes can be combined in a single or mixed layer. The substrate layer may be formed from a conductive material. Alternatively, a conductive layer can be formed on a nonconductive inert substrate by a technique such as but not limited to sputter coating.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer or other layer. For example, U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Imaging members with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life.
In the charge transport layer, the charge transport molecules may be in a polymer binder. In this case, the charge transport molecules provide hole or electron transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such a vinyl polymer, polysilylene or polyether carbonate, wherein the charge transport properties are chemically incorporated into the mechanically robust polymer.
Imaging members may also include a charge blocking layer(s) and/or an adhesive layer(s) between the charge generating layer and the transportive layer. In addition, imaging members may contain protective overcoatings. These protective overcoatings can be either electroactive or inactive, where electroactive overcoatings are generally preferred. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
Imaging members are generally exposed to repetitive electrophotographic cycling, which subjects the exposed charge transport layer or alternative top layer thereof to mechanical abrasion, chemical attack and heat. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed charge transport layer.
Although excellent toner images may be obtained with multilayered belt or drum photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers are developed, there is a greater demand on copy quality. A delicate balance in charging image and bias potentials, and characteristics of the toner and/or developer, must be maintained. This places additional constraints on the quality of photoreceptor manufacturing, and thus, on the manufacturing yield. In certain combinations of materials for photoreceptors, or in certain production batches of photoreceptor materials involved in the same kind of materials, localized microdefect sites (which may vary in size from about 50 to about 200 microns) can occur, using photoreceptors fabricated from these materials, where the dark decay is high compared to spatially uniform dark decay present in the sample. These sites appear as print defects (microdefects) in the final imaged copy. In charged area development, where the charged areas are printed as dark areas, the sites print out as white spots. Likewise, in discharged area development systems, where the exposed area (discharged area) is printed as dark areas, these sites print out as dark spots in a white background. All of these microdefects, which exhibit inordinately large dark decay, are called charge deficient spots. Such charge deficient spots can also occur in negatively charging photoreceptors, where a hole can be injected into the structure through the ground plane and carried up through the charge generating and charge transport layers.
Various protective coatings have been applied to both organic and inorganic photoreceptors. For example, U.S. Pat. No. 3,397,982 discloses an electrostatic imaging device comprising a photoconductive layer containing an inorganic glass material, and a photoconductive layer with an overcoating comprised of various oxides, such as germanium oxides, vanadium oxides, and silicon dioxides.
U.S. Pat. No. 3,655,377 discloses the use of an arsenic selenium alloy as an overcoating on a tellurium selenium alloy photogenerator layer. U.S. Pat. No. 4,420,547 discloses a layered photoreceptor having an ultraviolet light absorbing top layer.
Furthermore, there is disclosed in U.S. Pat. No. 2,886,434 processes for protecting selenium photoconductive substances with a thin, transparent film of a material having electrical characteristics comparable to selenium. Examples of materials disclosed as protective layers in this patent include zinc sulfide, silica, various silicates, alkaline earth fluorides, and the like.
U.S. Pat. Nos. 5,096,795 and 5,008,167 disclose electrophotographic imaging devices, where the exposed layer has particles, such as metal oxide particles, homogeneously dispersed therein. The particles provide coefficient of surface contact friction reduction, increased wear resistance, durability against tensile cracking, and improved adhesion of the layers without adversely affecting the optical and electrical properties of the imaging member.
U.S. Pat. No. 5,707,767 discloses an electrophotographic imaging member including a supporting substrate having an electrically conductive surface, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an optional anticurl back coating, a ground strip layer and an optional overcoating layer. At least one of the charge transport layer, anticurl back coating, ground strip layer and overcoating layer includes silica particle clusters homogeneously distributed in a film forming matrix.
U.S. Pat. No. 4,869,982 discloses an electrophotographic photoreceptor containing a toner release material in a charge transport layer. From about 0.5 to about 20 percent of a toner release agent selected from stearates, silicon oxides and fluorocarbons is incorporated into a charge transport layer.
U.S. Pat. No. 4,784,928 discloses an electrophotographic element having two charge transport layers. An outermost charge transport layer or overcoating may comprise a waxy spreadable solid, stearates, polyolefin waxes, and fluorocarbon polymers such as Vydax fluorotelomer from du Pont and Polymist F5A from Allied Chemical Company.
U.S. Pat. No. 4,664,995 discloses an electrostatographic imaging member utilizing a ground strip. The disclosed ground strip material comprises a film forming binder, conductive particles and microcrystalline silica particles dispersed in the film forming binder, and a reaction product of a bi-functional chemical coupling agent that interacts with both the film forming binder and the microcrystalline silica particles.
U.S. Pat. No. 4,717,637 discloses a microcrystalline silicon barrier layer.
U.S. Pat. Nos. 4,678,731 and 4,713,308 disclose microcrystalline silicon in the photoconductive and barrier layers of a photosensitive member.
U.S. Pat. No. 4,675,262 discloses a charge transport layer containing powders having a different refractive index than that of the charge transport layer excluding the powder material. The powder materials include various metal oxides.
U.S. Pat. No. 4,647,521 discloses the addition of amorphous hydrophobic silica powder to the top layer of a photosensitive member. The silica is of spherical shape and has a size distribution between 10 and 1000 Angstroms. Hydrophobic silica is a synthetic silica having surface silanol (SiOH) groups replaced by hydrophobic organic groups such as —CH3.